Tuners, or adjustable impedance transformers, are often used in microwave circuits and measurements to transform an impedance into another impedance such as a match (no reflections) or a complex conjugate impedance (maximum power transfer). Impedance tuners permit producing arbitrary terminations when optimizing or characterizing microwave devices, so they are essential in the design process of both the microwave components (such as transistors, diodes, amplifiers, mixers, MMICs, etc.) and systems. Through transforming impedances, tuners allow the improvement of device or system parameters (i.e., gain, power, noise, intermodulation distortion, adjacent channel power, etch).
The design processes for microwave circuits are extremely dependent upon the ability to accurately measure the characteristics of these circuits. To aid in this cause, a great variety of sophisticated measurement equipment has been generated In the past, different types of tuners have been developed to use waveguide media.
One known type of tuner is called an E-H tuner, and is described, for example, in P. I. Somlo and J. D. Hunter, Microwave Impedance Measurement, pp. 60-69, at p. 66, London, U.K.: Peter Peregrinus Ltd., 1985, and A. E. Bailey, Ed. "Reflections and Matching", Microwave Measurements, Second Edition, pp. 74-91, at p. 84, London, U.K.: Peter Peregrinus Ltd., 1989. This tuner includes a main waveguide junction section, having a waveguide junction at a common transverse plane with an E-plane and a separate H-plane waveguide section. Both of these transfer arms have an adjustable short circuit as termination; thus the E-H tuner can be used to match to any passive impedance. However, these types of tuners have several shortcomings and disadvantages. First, the magnitude and phase of the two arms are not independent, resulting in no simple way to tune arbitrary impedances. Second, they often have erratic tuning patterns. Furthermore, certain complex impedances cannot be obtained at all because of the resonances caused by imperfections in the mechanical tuning structure. Finally, due to the waveguide losses, these tuners have poor matching range (reflection coefficient of less than 0.9, but often less than 0.8) and high excessive dissipative loss (i.e., insertion loss in excess of what one would expect from the given reflection coefficient). Therefore, clearly the performance at higher frequencies is greatly degraded.
Another known tuner type is the slide-screw tuner, shown e.g. in G. L. Ragan, Ed. "Microwave Transmission Circuits", M.I.T. Radiation Laboratories Series, vol. 9, pp. 456-457 & 481-500, at p. 485, Massachusetts: Boston Technical Publishers, Inc., 1964; P. I. Somlo and J. D. Hunter, Microwave Impedance Measurement, pp. 60-69, at p. 63, London, U.K.: Peter Peregrinus Ltd., 1985; and A. E. Bailey, Ed. "Reflections and Matching", Microwave Measurements, Second Edition, pp. 74-91, at p. 80, London, U.K.: Peter Peregrinus Ltd., 1989.